looking at the photo it does look like mold (caused by the same bacteria that causes fungus)
was the holfreters ordered online or made by yourself?, sometimes the holtfreters available online is a modified version which uses less salt than actual holtfreters.
was the holtfreters in the water when the any of the tests performed as the kh is a bit low and the gh is very low, ideal kh is 3-8 higher being better, ideal gh is 7-14 higher being better.
use 100% holtfreters in the water containing your axolotl, which is (all measurements per 1 litre water) 3.5g salt, 0.2g bicarbonate of soda, 0.1g calcium chloride, 0.05g potassium chloride, also add 0.2g magnesium sulphate (epsom salt).
use methyline blue baths for treatment.
swab area with potassium permanganate.
as an added note when making holtfreters add the salt, calcium chloride, potassium chloride, magnesium sulphate to 500ml hot water (or 1ltr water depending on what bottle/container you have) and stir/shake till disolved, add bicarbonate of soda to a separate bottle/container of hot water and stir/shake till dissolved, wait till both solutions are cold then add solution 1 (salt, calcium etc..) to the tank (with filter running to allow for full dispersal) wait 30 mins then add solution 2 (bicarb), it isn't unusual to get slight cloudiness, this is temporary and is caused by the bicarbonate of soda combining with the calcium chloride to make calcium carbonate.